Basic Authentication - Some of my users are unable to login?

I am able to login to my website from a client computer using any of my admin accounts, but I can't seem to login with any of my other accounts.

I have even setup a user and group that have write/modify/read/execute priveleges on the folder & files that make up the website.

Any tips?

Note1: In Local Security Policy, my desired users are included in "access this computer from the network"
Note2: Using SSL encryption, but this was an issue before SSL
Note3: This was also an issue when I was using Integrated Windows Authentication
Note4: IIS 6
Note5: Windows Server 2003
Note6: This user does have a password, and I know what it is, and typed it correctly

Basic Authentication
Basic Authentication is a method of authentication that is supported by the majority of Web servers on the Internet. When a server uses Basic Authentication, the Web browser (or the FrontPage client) prompts the user for a name and password. The user name and password are then transmitted across HTTP, in clear text. Using this user name and password, IIS authenticates the corresponding Windows NT user.

Windows NT Challenge/Response Authentication
Windows NT Challenge/Response (also called NTLM) is a more secure authentication method than Basic Authentication. When a user is authenticated using Windows NT Challenge/Response, the user is logged on to the Web server computer as a network logon. The Web browser or FrontPage client first attempts to use the credentials the user supplied to log on to the client computer. If those credentials are rejected, Windows NT Challenge/Response will prompt the user for a name and password by means of a dialog box. If a user has logged on as a domain user on a local computer, the user won't have to be authenticated again when he or she accesses a remote computer in that domain.

there would be additional config and details please refer to this link

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